Heating tube for boilers and method of manufacturing the same

Heat exchange – With coated – roughened or polished surface

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Details

9051341, 427422, F28F 1318

Patent

active

060824441

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to a heat transmitting tube for a boiler having excellent adhesion controlling effect of depositions produced in the heat transmitting tube (solid substances precipitated when ingredients dissolved in a boiler water are boiled and evaporated in the tube) and a method of manufacturing the same, and more particularly it proposes a heat transmitting tube for a boiler which inhibits growth of depositions adhered onto an inner face of an evaporation tube in the boiler using a heavy oil such as heavy oil, residual oil produced in a petroleum chemical process, petroleum coke, asphalt or the like as a fuel.


BACKGROUND ART

The heat transmitting tube for the boiler is manufactured so as to efficiently contact with combustion gas of fossil fuel or high temperature process gas. For this end, the heat transmitting tube frequently contacts with various corrosive impurities contained in the gas such as sulfur oxide (SOx) and nitrogen oxide (NOx), or vanadium compounds (V.sub.2 O.sub.5, NaVO.sub.3, Na.sub.2 O.V.sub.2 O.sub.5 and the like) and sulfur compounds (Na.sub.2 SO.sub.4, K.sub.2 SO.sub.4 and the like) included as a combustion ash content, and so on and hence is liable to be chemically damaged. Particularly, the heat transmitting tube for the boiler burning a heavy oil fuel containing the vanadium compound and the sulfur compound is considerably worn out by accelerated oxidation corrosion resulting from the vanadium compound and sulfurization corrosion of the sulfur compound. These corrosion damages are called gas-side corrosion because they are created at the outer surface of the heat transmitting tube or a position contacting the combustion gas.
As a method of preventing gas-side corrosion, there has hitherto been proposed a method of forming protective coatings on the surface of the heat transmitting tube as mentioned below.
(1) In JP-A-61-41756 is disclosed a technique in which Ni--Cr alloy or self fluxing alloy is sprayed onto the surface of the heat transmitting tube for a fluidized bed type boiler burning coke and then fused by heating to impart heat resistance and abrasion resistance to the heat transmitting tube.
(2) In JP-A-60-142103 is disclosed a technique a self fluxing alloy coating is formed on the surface of the heat transmitting tube for a boiler covering waste heat in a dry type fire extinguishing device and fused by heating and further subjected to a solid solution treatment or an annealing treatment to prevent erosion.
The above two techniques are effective in boilers used under an environment in which the abrasion rate is larger than the corrosion rate.
(3) In JP-A-2-185961 is disclosed a technique in which Al is coated onto the surface of the heat transmitting tube for the boiler by spraying and a self fluxing alloy sprayed coating containing Al is formed thereon and then fused by heating to impart corrosion resistance to the heat transmitting tube.
(4) In JP-B-7-6977 and JP-B-7-18529 is disclosed the formation of a sprayed coating on the heat transmitting tube for the boiler.
As the corrosion damage created in the boiler, there is a water-side corrosion observed in an inner wall face of the heat transmitting tube or a surface passing a boiler water or an overheated steam therethrough in addition to the above gas-side corrosion. In general, the boiler water is usually adjusted to an alkalinity for controlling the above water-side corrosion. Therefore, as the operation of the boiler is continued over a long time period, an alkali component contained in the boiler water locally concentrates at the inner wall face of the heat transmitting tube and hence the tube material is corroded to produce an iron oxide. And also, compounds of Si, Ca, Mg, P, Cu and the like slightly contained in the boiler water precipitate on the inner wall face of the tube. As a result, obstruction of heat transmission is caused but also a phenomenon such as local overheating or the like is caused, and the heat transmitting tube is sometimes broken by these causes.
These phenomena

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